In the past, subsurface safety valves have had a variety of layouts of matched surfaces used for sealing. Curved flappers, when used in safety valves, allowed for a reduced wall or "slimline" safety valves. This feature has increased the popularity of curved flappers in subsurface safety valves. These have been either metal to metal or have involved some sort of resilient seal. The techniques previously used to cut such sealing surfaces have involved maintaining the flapper or flapper seat piece in a stationary position while the cutter was rotated about a radius from a fixed point. What resulted was that the seat and the flapper seating surface presented an undulating profile when extended into a flat plane. During the low segments of this undulating profile, the two surfaces were, in essence, in a plane transverse to the longitudinal axis of the seat or the flapper. However, taking off from those points, the seating surface and the mating flapper surface moved into a transverse plane or planes with respect to the longitudinal axis of the seat or the flapper. This type of profile is illustrated in FIGS. 1-3 which indicate the prior design. FIG. 1 illustrates a plan view of a seat or the matching surface on the flapper, while sections 2 and 3 are taken at different points along its continuous surface. The view of FIG. 1 illustrates the plan view looking down, with the X-axis transverse to the Z-axis as indicated. The Sections 2 and 3 are on the plane of the XZ-axis. FIG. 2 shows that at the low points of the surface, the sealing surface is essentially transverse to the Y-axis. However, this situation in the prior designs changes as one moves away from the point of Sections 2--2 toward Section 3--3, which is illustrated in FIG. 3. There the sealing surface is illustrated to be oblique with respect to the Y-axis. This occurs because the nature of how the cut is made either in the flapper or in the seat. In essence, the centerline of the cutter is only fully transverse to the seat at several points along the continuous surface while it is being cut and at other times is oblique to the surface being cut.
The drawbacks of the prior design are that it is not self-centering and it does not present as efficient a metal-to-metal contact as the present invention. Accordingly, the present invention has been developed to improve the quality of the metal-to-metal contact to provide a better sealing engagement to reduce or eliminate leakage. The objective is to reduce leakage rates even without the application of resilient seals. The addition of a resilient seal in the seat is easier to make with the new design. The techniques, as illustrated in the present invention, also have as an objective accommodating a design as a metal-to-metal mating, with or without the use of resilient seals.
The invention accomplishes these improvements by disposing the sealing surfaces throughout the entire 360.degree. periphery to be in a plane transverse to the longitudinal axis of the seat and flapper. This design facilitates the addition of a seal groove in the seat to accommodate a resilient seal. It also permits use of a stronger hinge since the machined surface of the seating face on the flapper is parallel to the inner hinge surface.